Help what happened

So I have set up my own mail server 🙃 🙂. What an achievement for me 😆😅 And it was working fine till recently. It becomes really, really slow. And I checked the log and found out this. What could I do and what's the purpose of this? Is it a brute force attack? All these ips are owned by the same person/group?? Any help or pointers will be deeply appreciated